Plant-derived glycogen, or “phytoglycogen” is a highly branched water-soluble polymer of glucosyl units. Plants utilize the plant polysaccharide in the same manner animals utilize glycogen: as energy for metabolic processes. A number of plants produce phytoglycogen, including corn, rice, sorghum, barley, and Arabidopsis. Phytoglycogen exists in particulate sizes of approximately 40 to 50 nm.
Plant starch biosynthesis depends on starch synthases, branching enzymes, and debranching enzymes. In the absence of debranching enzyme activity, phytoglycogen is formed instead of starch. In corn, a mutation in the “sugary 1” gene (su1) results in a deficiency of an isoamylase-type debranching enzyme, SU1. SU1-dysfunctional corn kernels are sweeter than varieties with functional SU1, due to an overabundance of phytoglycogens and accumulation of sugars.
The highly branched structure of phytoglycogen contributes to its unusually high molecular density when in dispersion. The dispersed molecular density of corn phytoglycogen is about 1200 g/mol/nm3, compared with about 60 g/mol/nm3 for amylopectin of starch. The natural density and branching of phytoglycogen promotes structural integrity and supports functional group grafting at the surface.
With regard to the polymer structure, phytoglycogen does not possess long chains that connect individual clusters, as does amylopectin. It is likely that phytoglycogen particulates grow from the non-reducing ends of glucan chains at the surface, by periodic branching and elongation of glucan chains.
Others have modified phytoglycogen for various purposes. For example, Scheffler et al. reported a procedure in which phytoglycogen was contacted with octenyl succinic anhydride (OSA) to yield phytoglycogen octenyl succinate (PG-OS). PG-OS showed an outstanding property to form oil-in-water emulsion after homogenization. Scheffler et al., J of Ag and Food Chem, 58: 5140-514 (2010) and Scheffler et al., J of Ag and Food Chem, 58: 660-667 (2010).
Bi et al. also reported procedures for reacting phytoglycogen with OSA or succinic anhydride (SA), yielding PG-OS and phytoglycogen succinate (PG-S) with various degree of substitution. These chemically modified phytoglycogen materials were further tested for their capability to adsorb antimicrobial peptide, such as nisin for prolonged antimicrobial effects against food pathogens such as Listeria monocytogenes. Bi et al., Biotech and Bioengineering, 108: 1529-1536 (2011) and Bi et al., J of Controlled Release, 150: 150-156 (2011).
In these reports, however, chemical substitutions were used to bring functionalities to phytoglycogen materials. This not only increases the cost of products, but also is unsuitable for the natural and “green” formulations that are highly pursued by the industry and consumers.
Previously-known food emulsifiers include proteins (e.g. casein, whey protein, and soybean proteins), small-molecule surfactants (e. g. lecithin, sorbitan esters, sugar esters, and monoglyceride), and polysaccharide-based materials (e.g. gum Arabic, starch octenyl succinates, and hemicellulose). The primary use of these emulsifiers is to stabilize oil-in-water or water-in-oil droplets by forming a stable interfacial layer. For the purpose of encapsulation, the emulsions (usually the oil-in-water emulsions) are converted to a solid form by methods such as spray drying or freeze drying. Usually the wall materials or bulking agents are needed for encapsulation to form a protective layer over the oil droplets. Biopolymers, particularly those with low viscosity at higher concentrations, such as maltodextrin are suitable as wall materials. Gum Arabic and starch octenyl succinate (starch-OSA), both having emulsification and bulking properties, are suitable for encapsulation.
Starch-OSA is chemically modified, which limits label claims desired by the food industry. Hemicellulose has high cost and high viscosity that hinders its applications in encapsulation.
Protein-based emulsifiers are broadly used; however, they have disadvantages of high cost and low heat stability in food processing.
While polysaccharide-based emulsifiers can form stable emulsions, there are limitations for their applications. For example, gum Arabic faces the challenge of high cost due to scarcity of this material.
Therefore, there is a strong need for methods for preparing natural and functional emulsifiers without using chemical modifications.